In offset printing an image is transferred from an inked plate to a blanket having an elastomer printing surface, and from the blanket to the paper being imprinted. Typically, an offset printing blanket has an elastomer body with one or more layers of fabric reinforcing and a fabric backing. The blanket is stretched around a cylinder and the ends are secured by a locking mechanism located in a longitudinal gap in the cylinder surface. Due to the planographic nature of offset printing, high pressure is required between the blanket and the plate or paper to insure transfer of the ink image. The discontinuity of the cylinder gap causes problems in high speed web printing, affecting the quality of the printed product and the design and maintenance of the press. For example, cylinders typically have bearer rings which minimize impact and vibration attributable to the locking mechanism gap, but which require a substantial mechanical load, introducing bearing and wear problems. Moreover, the gap leaves an unprinted area on the paper web, resulting in paper waste.
A magnetic gapless cylinder for web offset presses is shown in Peekna application Ser. No. 736,062 filed May 20, 1985, now U.S. Pat. No. 4,625,928, and Peekna et al. U.S. application Ser. No. 763,128, filed Aug. 6, 1985, now Pat. No. 4,676,161 assigned to R. R. Donnelley & Sons Company. This application discloses and claims a printing blanket on a carrier plate of ferromagnetic material suitable for use with the Peekna cylinder in web offset printing, and a method of assembling the blanket. Banike U.S. application, Ser. No. 642,080, filed Aug. 20, 1984, and assigned to R. R. Donnelley & Sons Company, now U.S. Pat. No. 4,577,560, discloses a gapless lockup mechanism for a plate cylinder. A blanket mounted on a plate can be used with a cylinder which has the Banike lockup mechanism.
Adhesive coated "sticky back" blankets which mount directly on a cylinder are used in offset form printing. The adhesive is dissolved by fountain solution and the "sticky back" blankets are not suitable for web offset printing. Others have mounted an elastomeric printing sheet on a steel plate, but not for the severe physical and chemical environment to which a blanket is subjected in offset web printing. For example, Faust U.S. Pat. No. 4,040,351 shows a rubber printing mat cemented to a steel base of shim stock, mounted on a magnetic cylinder, in a flexographic labeler or addresser. McKay U.S. Pat. No. 3,180,259 shows a molded rubber printing plate cemented to a steel base plate held on a magnetic printing wheel as used in a coding or dating machine. Jenkins U.S. Pat. Nos. 3,885,497 and 3,885,498 show molded magnetic cylinders, on which printing plates are mounted, and methods for molding the bases. Stromme U.S. Pat. No. 2,982,207 has a flexible printing plate secured, as by bonding, to a corrugated plate.
None of these blankets or resilient plates is subjected to the pressures, speeds or chemical exposure of web offset printing.